This invention relates to a pig iron refining process and to the resulting product, in which an oxidizing gas is injected, such as, e.g., industrially pure oxygen, to eliminate carbon or other oxidizable impurities. More specifically, the invention is directed toward a process and product in which all or part of the oxidizing gas is injected under the surface of a bath of the molten metal. Processes of this general type are mainly known under the names of OBM, QBOP and LWS, when referring to those in which the larger part of the oxygen is blown in from below, and under the names LD-OB, LD-OTB and STB, when referring to those in which only a small part of the oxygen is injected from under the surface of the bath.
In one of the more commonly used processes for producing steel pneumatically, oxygen is blown through a nozzle above the load, in such manner that the jet of oxygen penetrates the molten mass and forms very oxidized slag that, upon contact with the pig, reacts with carbon to produce carbon monoxide. In processes blown via the bottom, oxygen is injected under the surface of the bath through nozzles located in the bottom or near the bottom of the converter. A protecting gas, generally a hydrocarbon or a non-oxidizing gas (that may be in liquified form) is used to surround the current of oxygen in order to reduce wear, a very important factor in the case of nozzles as well as refractory elements in the bottom of the converter. One of the considerable advantages of the latter processes in comparison with the former is the ability to obtain higher metallic yields. These yields are mainly achieved because:
1. the oxygen traversing the metallic bath stirs up the bath more intensely and allows a greater approach to equilibrium conditions, and
2. the amount of iron oxide fumes produced is much smaller since the carbon's oxidation reaction is located in the very essence of the metal, contrary to the refining processes from above in which this reaction takes place at the slag-metal interface. It follows that refining processes from above are inadequate to obtain, under good conditions, low and very low carbon content steel.
New processes have attempted to mitigate this drawback: e.g., the LBE and LDAB processes, in which a neutral gas favoring the rabbling of the metal is injected through the bottom, while not going as far as the processes in which part of the oxygen is injected through the bottom. However, these refining processes through the bottom have so far not made it possible to obtain, in an oxygen converter, low or very low carbon content steel which is not high in dissolved gas content, mainly oxygen.
Nevertheless, refining processes through the bottom yield the lowest dissolved oxygen content compared to refining processes from above.
The presence of dissolved oxygen in the liquid metal is particularly bothersome. When the metal solidifies, this oxygen reacts with oxidizable elements and more specifically with the residual carbon to form CO. The result is a lower carbon content in the solid metal, a lack of homogeneity due to the presence of cavities containing carbon monoxide and, above all, in the case of extra-soft steel, the presence of metallic oxides.
There are several processes which attempt to remedy these drawbacks. The first of these techniques is that called killing. Highly oxidable elements such as aluminum, silicon and other metalloids or mixtures of the latter are added to the liquid metal, before casting in ingots or continuous casting. The elements react with dissolved oxygen to form oxides that decant and are trapped by the covering slag. Although there still remains a certain amount of these oxides in the metal when it solidifies, the morphology of the inclusions is controlled more adequately.
Another technique, used in a converter, purifies the metal with the help of a neutral gas, mainly nitrogen or argon. Its drawbacks are that it is only moderately effective and that it changes the carbon content of the bath, leading to a greater dispersion of carbon content in the casting.
These latter techniques may be grouped under the generic term of vacuum treatment techniques. Such techniques in general perform well, but they have the following additional drawbacks:
1. large investments;
2. high operational and maintenance costs due to the procedures used for obtaining a vacuum;
3. temperature losses requiring either overheating during casting, or a system to reheat the molten mass;
4. long processing time.
In the processes in which a gas containing oxygen is blown through a nozzle located under the surface of the bath, refining takes place in two stages:
1. Formation of a microslag mainly containing iron oxide according to the reaction:
Fe+O FeO PA0 FeO+C CO (gas)+Fe PA0 x=-766.7 %C+168.7 for 0.16&lt;%C&lt;0.22 PA0 x=-550%C+134 for 0.1&lt;%C&lt;0.16 PA0 x=-233 %C+102 for %C&lt;0.1 PA0 x=-1500 %C+255 for 0.14 %C 0.17 PA0 x=-400 %C+101 for 0.1 %C 0.14 PA0 x=-230 %C+84 for 0.05 %C 0.1
2. Decantation and reduction of said microslag: as it rises through the metallic mass this slag reacts with the carbon of the bath according to the reaction:
During refining, two stages may be distinguished:
1. An initial stage in which the bath contains sufficient carbon to reduce all the iron oxide produced: this occurs when the carbon content of the bath is above a certain value C*.
2. A second stage in which the carbon content in the metallic mass is too low to reduce all the iron oxide produced at the tip of the nozzle, leading to a notable reduction in the iron yield of the refining operation and an increase in the amount of iron oxide contained in the slag.
U.S. Pat. No. 3,930,843 describes a refining process through the bottom in which a mixture of oxygen and argon is introduced, through the bottom of the converter, into the molten steel bath, when the carbon content of said steel is lower than 0.25%. This introduction is carried out according to a process that includes three successive phases of dilution of the oxygen by the argon according to the carbon concentration in the metal bath. This patent gives no indication on how to obtain the desired steel concomitantly with a reduction in the duration of the refining process and the consumption of argon.
In French Pat. No. FR-A-2 448 572 a refining process for refining low carbon content steel in a converter is described, in which argon is introduced with the oxidizing gas as of a predetermined value of carbon content, in this case 0.02%. However, this value is too low to obtain low dissolved oxygen content steel. For such a value, the dissolved oxygen concentration is very important, and an injection of neutral gas cannot lower said content in an effective manner.